The Best Polycarbonate Greenhouse Kits

The sensible first greenhouse: an aluminum frame with twin-wall panels, sized to fit a normal backyard rather than a nursery.

Twin-wall polycarbonate is the thing that separates a greenhouse from a plastic tent. Two skins with an air gap between them insulate, and the panel diffuses light rather than throwing hard shadows, which suits seedlings better than clear glass does.

The Mythos sits at the point in the range where you get a proper aluminum frame, a roof vent and a lockable door without paying for the heavy-duty glazing. For starting seeds, hardening off, and stretching the tomato season at both ends, that is the right specification.

Two warnings from the manual that people ignore. Anchor it -- every lightweight greenhouse that ends up in a neighbor's yard was not anchored. And build it with a second person; the panels flex and the frame is not square until the last bolt.

Skip it if: you live somewhere with heavy snow load. 4 mm panels are a three-season specification; the thicker-glazed models exist for a reason.

The one to buy if you want a premium greenhouse for what it withstands rather than what it looks like: published ratings, heavy frame section, lifetime frame warranty.

Grandio publishes the two numbers that decide whether a greenhouse survives where you live -- 76 mph of wind and 25 lb per square foot of snow -- and backs the frame for life. Very little else in this bracket does both, and a kit that quotes no rating at all is telling you something by omission.

The glazing is 6 mm twin-wall, which is where expectations need managing: at this money you might assume 10 mm. What the price buys is frame section, fastener quality, truss reinforcement and the warranty behind them. If overnight heat retention matters to you more than structure, a thicker-glazed kit for less money is the better buy -- and that trade is the whole argument of the roundup this sits in.

Budget for the anchor kit and for ground preparation. Concrete slab, wooden deck and soil anchor kits are each sold separately, and the rating only applies to a structure that is properly fixed down.

Skip it if: you want a decorative garden building. This is an honest piece of engineering in powder-coated aluminum, not a feature of the yard.

The heavy-glazed model in the range, and the one to buy if the plan is to keep growing through a real winter.

Glazing thickness is the whole argument for this kit. A 10 mm twin-wall panel holds substantially more heat overnight than a 4 mm one, and overnight is when a winter greenhouse either works or does not.

It is also the model to look at if you get real snow. Thin panels and light frames are a three-season specification; if you are in the Cascades or anywhere with a genuine snow load, buy the heavier structure rather than hoping.

None of that makes it heated. A greenhouse without a heat source holds a few degrees over outside, which is the difference between hardy greens surviving and not -- it is not a tropical house in January.

Skip it if: you only want a spring seed-starting space. You are paying a large premium for insulation you will use for three months of the year.

Clear walls so you can see what is happening inside, with a diffusing twin-wall roof over the plants that need protection from hard overhead sun.

The Hybrid name refers to two glazing types in one kit: crystal-clear panels on the walls, twin-wall on the roof. In practice that means high light levels from the sides and diffused light from above, which is a good compromise in a northern or cloudy garden.

It also means you can see in. That sounds trivial until you have walked out in the rain three times a week to check whether anything needs water.

In a hot, south-facing, unshaded spot the clear walls become a liability. If that is your site, choose the fully diffused model or plan on shade cloth from midsummer.

Skip it if: your site bakes in afternoon sun. Clear walls transmit far more light, and heat follows light.

A barn-style hobby house that buys room to walk, stand and bench down both sides -- as long as you read the glazing spec before you commit.

Floor area is what this kit sells, and 144 square feet changes what a greenhouse is for. You can bench both long sides, keep a path down the middle, stand upright under the ridge anywhere, and get a barrow in through the double doors. Most hobby kits make you pick two of those.

The glazing mix is the thing to understand before buying. The roof is 4 mm twin-wall, which diffuses light and holds a little heat; the sides are 0.7 mm clear panels, which are a window rather than insulation. That is deliberate -- light in, view out -- and it is a poor match for winter growing.

The published ratings follow from the build: 56 mph of wind and 15.4 lb per square foot of snow. In a mild, sheltered yard that is fine. In the Cascades, or anywhere a foot of wet snow is normal, buy the structure that is rated for it instead.

Skip it if: you are buying for winter growing or live under real snow. A 15.4 lb per sq ft snow rating and 0.7 mm side panels are a three-season specification.